Why am I getting a 403 Forbidden error on Google Webfonts in my custom theme?

[mike-sod]

Our Shopify Theme (https://justplanet.com.au/) uses Google Webfonts. Within the theme editor, they load fine, but as of about a week ago on the front end, we get 403 forbidden errors from the Shopify CDN.

I've tried contacting Shopify support, but they said it's a custom theme and can't help, but we are using the fonts prescribed in the Shopify docs.

Any help in resolving this would be greatly appreciated.